In inkjet printing systems, it is desirable to control and maintain the print density of the printed images over time. The print density is influenced by the optical density of the inks used. As the optical density of the inks depends on the concentration of the ink, it is common for the printing system to include an ink concentration sensor. To provide additional control on the print density, especially in color printing systems, the printing system can include sensors for measuring print density of the printed images. It has been found that both ink concentration sensors and print density sensors can fail, allowing the print density to deviate from normal. An improved control system is needed to overcome this problem with prior art systems.